Micromachining with femtosecond laser pulses, in which the transient generation of a plasma leads to the ablation of material, is a powerful technique to cut chemically inert media such as glass. This procedure uniquely facilitates the prototyping of three-dimensional (3D) microanalytic devices with sub-diffraction-limited features. However, single-step processing has been limited in the size and aspect ratio of the features that can reasonably be produced in these media. As examples, in most fabrication techniques a laser beam is focused on the front surface of the substrate and ablation proceeds from the top down. Thus, successive pulses must focus through debris created by earlier pulses, and the pulses ultimately interact with the walls of the structure as the feature becomes deeper. This leads to a tapering of the feature that limits the aspect ratio.
An improved machining method would enable processing to take place through the backside of the wafer. Machining in this manner means that successive pulses would no longer focus through debris, nor interact with the walls, and thus makes it possible to produce exceptionally high aspect ratio features. Some systems have been designed which ablated high aspect ratio structures on the back surface of 1 mm thick glass at high numerical aperture (0.55 NA). The working distance of these original systems has been extended by employing a long working distance objective at 0.42 NA.
Other systems have achieved high aspect ratio structures with Bessel beams by focusing on the backside of the substrate. However, Bessel beams do not have the same 3D control as other techniques.
An improvement to backside machining would be to use lower NA beams to increase the interaction volume but without compromising 3D spatial confinement.